Use of lead coatings on pistons

Something I had never come across before (that's not to say it isn't in common use!) is the use of lead coatings on pistons by Jaguar on one of their prototype quad-cam V12 engines.

Why would they do that? Surely the environment in the compression chamber is too harsh for lead? What are the benefits if any?

The original 1966 build log clearly shows the following:

During the stripdown of my engine I was surprised to see a white "bloom"/powder on the crowns of the pistons (the engine had lain untouched for 40 years or so) which did have the appearance of lead oxide. Although the white deposits were quite thin and easily removed simply by rubbing, the piston crown still has a dull finish which could be a lead coating? The coating seems to be limited to the piston crown.

Here is a picture of the pistons before they were removed from the engine:

By way of comparison, here are the cleaned-up pistons temporarily reinstalled in the engine:

The picture doesn't show it very well but the crowns have quite a dull finish.

I am sure there are simple tests available for lead, in fact your local hardware probably has a cheap kit for testing old paintwork for lead content. Lead coating might provide an anti-sieze benefit in the skirt or land areas in the event of lube failure.

I am sure there are simple tests available for lead, in fact your local hardware probably has a cheap kit for testing old paintwork for lead content. Lead coating might provide an anti-sieze benefit in the skirt or land areas in the event of lube failure.

Thanks gruntguru. However, I don't need a test as I am pretty sure it is lead. Yes - my first thought was that there could be a lubricant effect but the coating is only on the top of the crown.

I am also puzzled by the fact that the minimum combustion chamber temp is around 100 degrees C more than the melting point of lead. Do small amounts of lead vapour have any anti-knock properties?

What they are calling "lead" here might be a bit of a colloquialism. The coating might be more accurately described as terne or solder -- that is, some alloy of lead and tin in widely varying proportions. In the USA, this same general piston treatment was traditionally called "tin" even though, strictly speaking, it was a similar sort of tin/lead alloy. The dull finish on the pistons above indicates a lead-heavy alloy pretty close to old-fashioned solder -- 70/30 or 80/20 lead/tin. Nearly all OE pistons were tin/lead plated at one time, especially on the skirts, but most usually with a greater percentage of tin, producing a lighter gray color closer to plain aluminum. As industry continues to distance itself from the metal, the lead in this process has been replaced with other materials, often zinc.

The purpose of these coatings is 1) corrosion resistance and 2) dry-running protection on start-up.

With iron liners it is common to coat the aluminum piston skirts with a soft coating to minimize scuffing. The current coating of choice is molybdenum disulfide dry film lubricant. But in 1966 MoS2 DFL was not widely available. Silver plating would have also been an excellent choice, but it would have been expensive. Lead works almost as well as silver and is far less expensive.

The lube oils used in 1966 also were formulated to accommodate lead deposits, since TEL was still used in gasoline as an octane booster.